There has recently been considerable research into organic multi-layered thin film light-emitting devices which emit light when electrons injected from a cathode and holes injected from an anode recombine within an organic fluorescent body interposed between the electrodes. Such devices are a focus of attention on account of their characteristics of thin shape, high luminance at low driving voltage and polychromic light emission based on suitable selection of the fluorescent materials.
Numerous research organizations have been carrying out such research since C. W. Tang and co-workers at Kodak first described the fact that an organic multi-layered thin film element emits light of high luminance (Appl. Phys. Lett. 51(12) 21, p. 913, 1987). A typical organic multi-layered thin film light-emitting element construction proposed by the Kodak research group is that in which there are provided, in turn, on an ITO glass substrate, a hole transporting diamine compound, 8-hydroxyquinoline aluminium as the emissive layer and Mg:Ag as the cathode, and 1,000 cd/m2 green coloured light emission is possible at a driving voltage of about 10 V.
In this organic multi-layered thin film light-emitting device structure, as well as the aforesaid anode/hole transporting layer/emissive layer/cathode, there may also be suitably provided an electron transporting layer. The hole transporting layer has the function of transporting the holes injected from the anode to the emissive layer, while the electron transporting layer transports the electrons injected from the cathode to the emissive layer. By interposing these layers along with the emissive layer between the two electrodes, the luminance efficiency and the durability are enhanced. As examples of device structures employing these, there are structures comprising an anode/hole transporting layer/emissive layer/electron transporting layer/cathode and an anode/emissive layer/electron transporting layer/cathode, etc.
However, many conventional emissive materials, hole transporting materials and electron transporting materials lack durability, and crystallization takes place due to the heat evolved from the device by the prolonged passage of current, so that the device life is shortened.
In particular, taking the case of the electron transporting material, with many of the existing materials there are problems such as the desired emission colour not be obtained for reasons such as interaction with the emissive material or there being admixed light emission by the electron transporting material itself, while even where highly efficient emission is obtained the durability is poor. In U.S. Pat. No. 5,393,614, a specific phenanthroline derivative is used as the electron transporting material, but while highly efficient emission is shown there is crystallization during prolonged operation, and the thin film turns cloudy. Quinolinol metal complexes and benzoquinolinol metal complexes are also materials which show comparatively good characteristics in terms of luminance efficiency and durability but, since these materials themselves have a high emissive capacity in the blue-green to yellow region, when employed as electron transporting materials there is admixed emission from these materials themselves and the chromatic purity is adversely affected.
The present invention has the objective of resolving such problems of the prior-art and offering a light emitting device which is excellent in its thermal stability, has high luminance efficiency, high luminance and excellent chromatic purity.